Meristems are found at the tips of roots and shoots. Here, unspecialised stem cells undergo a type of cell division called mitosis. This does not occur in other parts of the plants. In all other parts of plants, cells can only become bigger in a process called cell elongation.
Cells produced by mitosis in meristems help plants continue to grow in height and width throughout their life. Animals do not have meristems, so they stop growing in size once they become adults.
Plant meristems divide to produce cells that increase the height of plants, the length of roots and the girth of the stem. They also produce cells that develop into leaves and flowers. So they can differentiate into specialised cell types depending on where they are in the plant.
As multicellular plants, like multicellular animals, increase in size, their smaller surface to volume ratio becomes smaller. Because of this, they have evolved tissues, organs and organ systems. As in animals, these are specialised.
Together, the roots, stem and leaves form a plant organ system for transporting and exchanging substances around the plant.
Plants take in water from the soil, along with dissolved ions such as nitrate - which they use to make proteins - and magnesium - which they use to make chlorophyll. These are transported in the roots, stem and leaves.
Any part of a plant above ground, where it is exposed to light, can photosynthesise. But a plant's leaves are the main organ of photosynthesis.
Plant leaves are adapted for gas exchange and photosynthesis.
|Large surface area||For maximum absorption of light; for optimum absorption of carbon dioxide|
|A thin, flattened blade - though there are exceptions||So that water and gases have a limited distance to diffuse|
|Vascular tissue in the midrib and veins||Branches of the xylem, to transport water and minerals to the leaf; branches of the phloem, to transport glucose and other products of photosynthesis to other parts of the plant|
|Air spaces between cells within the leaf||For the diffusion of gases|
Plant leaves are adapted for photosynthesis, and the exchange of gases required for the process. The structure of the tissues is related to their functions in the plant.
The palisade mesophyll layer of the leaf is adapted to absorb light efficiently. The cells:
Spongy mesophyll tissue is packed loosely for efficient gas exchange. The spongy mesophyll cells are covered by a thin layer of water. Gases dissolve in this water as they move into and out of the cells.
When the plant is photosynthesising during the day, these features allow carbon dioxide to diffuse into the spongy mesophyll cells, and oxygen to diffuse out of them.